Transistorized amplifier utilizing an input transformer bootstrap configuration



Aprll 13, 1965 DE Loss J. TANNER 3,178,648

TRANSISTORIZED AMPLIFIER UTILIZING AN INPUT TRANSFORMER BOOTSTRAPCONFIGURATION Original Filed Jan. 8. 1960 INVENTOR.

United States Patent ice This invention relates to electronicintercommunication apparatus and more specifically to a novel transistorcircuit therefor. This application is a continuation of the applicationfiled January 8, 1960, entitled Transistorized IntercommunicationSystem, and assigned Serial No. 1,360, now abandoned.

A general object of the invention is to provide a low cost, batteryoperated, transistorized intercommunication device which can be easilyinstalled and operated by nontechnical personnel.

A further object is to provide a circuit which is cost and performanceoptimized so that it provides exceptional performance at very low cost.

A different object is to provide a circuit which is battery operated andswitched so that power is utilized only during use, which should prolongthe life of the battery to a point where under normal usage it wouldapproach shelf life.

Another object is to devise a circuit which uses low impedance speakersfor economy and to minimize hum pickup between the main and subunits toa point where the signal leads do not have to be shielded.

A diiferent object is to provide a circuit in which the inputtransformer is utilized in a bootstrapping arrangement to eliminatecoupling components between the two transistor stages in the circuit.

A still further object is to provide a simple volume control which, inaddition'to controlling volume also controls battery drain.

A still further object is to provide, without adding special components,a novel amplifier circuit incorporating series current limiting toreduce the possibility of transistor failure through overloading.

munication apparatus made in accordance with the invention;

FIGURE 2 is a schematic circuit diagram of the invention.

Describing one embodiment of the invention in detail and havingparticular reference to the drawings, there is shown a two stationintercommunication system comprising a main unit generally designated 2housed in a suitable casing 3 and comprising a combination speaker andmicrophone 4 which, in this embodiment, has a low impedance 3.2 ohmvoice coil 4' with a grounded lead 5 and a lead 6 which is connected tobranch lines '7 and 8, the line 7' oeing connected to a contact 9 of aswitch 10 and line 8 to a contact 11 of switch 10.

The switch 10 is a double throw double pole spring return type and hasits pole member 12 connected to one lead 13 of the primary winding 14 ofan input transformer 15, the winding 14 having its other lead 16grounded.

The transformer has its secondary winding 17 connected through one lead18 to the base 19 of a first transistor 2t) and the opposite lead 18' ofwinding 17 is connected through a resistor 21 to ground and to thenegative lead 22 of an electrolytic condenser 23, the positive lead 24of condenser or capacitator 23 being connected to one end 25 of a volumecontrol resistance 26 which isconthe main unit is depressed.

3,173,648 Patented Apr. 13, 1965 nected by its variable arm connection27 and at the other end to a lead 28 which connects to the emitter 29 ofthe transistor 20. The collector 30 of transistor 20 is grounded. Thelead 18 is also connected through the resistor 32 to the positive lead33 of the battery 34, and this lead 33 is also connected through aresistor 35 to the line or lead 36 which is connected to the base 37 ofthe second transistor 38, the emitter 39 of which is connected to thelead .40 which connects to one end of a resistor 41 which at its otherend is connected by lead 41 to the positive battery lead 33. The lead 40is also connected to the positive side of an electrolytic condenser 42which has its negative side grounded through lead 43. Those skilled inthe art will recognize that other battery connections can be used, withappropriate circuit connections to apply the requisite operatingpotentials throughout the circuit.

The collector 44 of the transistor 38 is connected through lead 45 tothe primary winding 46 of an output transformer 47, the winding 46 beingconnected through lead 48 to ground.

The secondary winding 50 of transformer 47 is connected through lead 51to ground and through lead 52 to pole element 53 which in its normalposition engages contact or point 11 of switch 10.

The pole element 53 is adapted to contact the point 56 which isconnected through line 57 to contact 58 of switch 10 which connects tolead 13 of the input transformer primary winding 14.

Contact 56 is also connected to line 59 which is connected to one end ofvoice coil 60 of the speaker-microphone 61 of the sub unit 62, the coil60 being connected through lead 63 to ground and through line 64 to asingle pole single throw switch 65 which is connected through line 65 toa single pole single throw switch 66 to ground and before the switch 66to the lead 67 which is connected to the negative side of the battery34. If desired, the switching arrangement in the main unit can be athreeposition switch, biased to return to the neutral position 'fromeither the talk or listen position.

It will be understood that the switching arrangement 65' between themain and sub units. This was accomplished by having what would normallybe only a simple return on-olf switch 65 in the sub-unit serve also as apush-to-talk switch.

The second spring return switch 66 in the main unit, also connected as apower on-off switch, doubles as a push-to-listen switch when the push tolisten button in It is also actuated when the push-to-talk button (inthe 'main unit) is pressed. However, the main units push-to-talk buttonalso actuates another switch 10 which interchanges (in an electricalsense) tween the main unit and subunits will not be subject -toexcessive hum pickup. Hum pickup is so low in fact, that it is notnecessary to shield the interconnecting leads. The disadvantage of lowimpedance speakers of requiring both input and output transformers forreasonable efficiency is largely overcome by a special amplifier circuitwhich utilizes the input transformer in a bootstrap configuration toeliminate coupling components between the two transistor stages.

OPERATION OF THE DEVICE Assuming that the subunit initiates a call theswitch 65 is depressed connecting the negative end of the battery. .34to the ground system. The electron flow is from the battery 34 throughlead 65', switch 65 and lead 63. The main unit circuit is energized, andthe operator then speaks into the speaker unit 61 while holding theswitch 65 closed. The speaker 61 generates a small fluctuating voltagecorresponding to the users voice. The signal voltage travels throughlead 59 and lead 63 to the main unit 2 where it reaches the primary 14of the input transformer 15 via part of switch through contacts 12, 58.The voltage is stepped up by the transformer and fed into the inputelectrode or base of the first transistor which is connected in abootstrap configuration, that is, the secondary 17 of the inputtransformer 15 returns to the top end of the major emitter resistance 35(through a DC. blocking capacitor 23) rather than the bottom end ofresistance 35, so that the resistance 35 will not introduce emitterdegeneration with consequent loss in gain.

The first stage may be considered a common emitter amplifier which hashad its collector load moved to the opposite end of the battery. Theresult of this arrangement is that a voltage amplification is achievedwhich would not be possible with a conventional emitter follower circuitusing a junction transistor.

It will be noted that if a conventional common emitter stage were usedin place of the circuit shown, the DC. voltage on the first transistorscollector would be too high to correctly bias the output stage base. Acoupling capacitor would therefore have to be added to block DC. and twoextra resistors (for biasing the output base) would be necessary.Besides the added cost the power taken by such resistors would causegain loss and add to battery drain. A second reason for the specialcircuit shown in the first stage is that the DC. current amplificationprovided by transistor 20 is used to supply a stable DC. bias for theoutput base 37 and only one set of base biasing resistors 32 and 21 isneeded for both stages. The second stage 38 is indirectly biased by 32and 21 through 20, which is a conventional emitter follower in so far asDC. is concerned. The efifect of current amplification of 20 is the sameas though much lower value base biasing resistors were used for theoutput stage 38. In addition to saving parts, the battery drain islowered (low resistance output base biasing resistors would increase thebattery current drain). Good bias stability is necessary in transistorcircuits to overcome the effects of temperature changes which causevariations in transistor characteristics.

The 1000 ohm resistor 35 was empirically chosen to produce maximum gain.Increasing or decreasing the value of this resistor more than 30%results in a noticeable gain loss.

The 500 ohm variable resistor 26 is inserted in series with the emitterof transistor 20 to serve two functions. First it is a simple volumecontrol which reduces gain as the resistance is increased (this can bereplaced by a fixed or set gain resistor or by a short) because ofemitter degeneration introduced. It also has an effect on the outputbase bias 37 and therefore an effect on the output collector 44 current.The collector current at 37 drops as the resistance of the volumecontrol resistance 26 increases, providing a battery saver" feature withlow volume settings. There are two advantages of the emitterdegeneration type gain control as used here over the conventionalpotentiometer volume control hook up. The first is that the control doesnot introduce an insertion loss as the inclusion of a potentiometeracross the secondary winding 17 would do and second, the minimum gainthat the control can be set to is purposely limited. This isaccomplished without adding a resistor, as would be necessary with apotentiometer hook up. The minimum gain is limited so the control cannotbe turned so low that parties at either end of the system cannot calleach other.

The output stage 38 is a single ended class A common emitter connectedtransistor with a 47 ohm emitter resistor 41. The value of resistor 41was chosen in conjunction with the values of resistors 21, 35, 32, theoutput transformer 47, and the supply voltage to provide as much poweroutput as is practical consistent with low battery drain and good biasstability. Note that the emitter by pass capacitor 42 does not go acrossthe emitter resistance, but connects to ground instead. The primaryreason for this is to obtain protection against damage to the outputtransistor by the high initial surge current that would otherwise bepresent immediately after power is applied and before the capacitor 42has a chance to charge up.

The positive end of the battery is not connected to either transistor orto any capacitor but must go through the resistances 32, 35 and 41. Thesurge current limiting these unbypassed resistors provide isparticularly important in the present system because the power will beturned on and off many times during normal operation. This innovationwill result in substantial improvement in reliability by minimzng thepossibiltiy of transistor breakdown.

The value of the output stages emitter bypass 42 is not excessivelylarge (considering its location in the circuit) in that the lowercapacity electrolytics are lower priced and second the value wasselected to reduce hum and low frequency noise, Without being low enoughto affect voice frequencies.

The audio output transformer 47 matches the relatively high outputimpedance of 38 with the 3.2 ohm speaker impedances.

Intercom switching As indicated earlier, the switching arrangement hasbeen greatly simplified, especially if the following is considered:

A. The only time power is taken from the battery is when someone pushesone of the spring return buttons.

B. Only three interconnecting leads are needed between the main and subunits.

C. The main unit has a press to listen button in addition to the pressto talk button. This is what might be called an eavesdropping feature.

To simplify the discussion of the switching, it is convenient to dividethe switching functions into two general categories-viz, speakerswitching and power on-ofi. switching. A D.P.D.T. switch is used as aspeaker switch. This is a spring return type which is connected so thatthe sub unit speaker is normally hooked to the amplifier input and themain unit speaker to the amplifier output. When this switch isdepressed, the main unit speaker is connected to the input and the subunit speaker to the output.

The two power on-otf switches 65 and 66 are connected in parallel andplaced in series with the battery so that either can connect the batteryto the amplifier when actuated. Note that one of the leads to the on-olfswitch (push to talk) in the sub unit is also used as one of the speakerwires. Without this sharing of the black lead 63, four wires would benecessary between units instead of three.

Hookups for difierent modes of operation 'unit does, except that sincethe operator is at the main unit,

he will hear sound originating at the sub unit.

(3) Press to talk switch in the main unit is pressed.- Since the speakerin the main unit must be the microphone for this mode of operation, theD.P.D.T. switch is actuated to exchange the position of the speakers inthe system. This alone is not enough to make the system work, however,because the battery would not be connected to the amplifier.Accordingly, when the push to talk button in the main unit is pressed,it also actuates switch 66, which turns the unit on.

LIST OF PARTS Reference Item Description Numerals 4 and 61. Speakers 3.2ohms. Ganged Double Throw Double Pole Spring Return Switch. InputTransformer 3.2 ohms to 2,000 ohms. and 38.--- Transistors..- R1OAt2N27Oor equivaen 21 Resistor 47K ohms. 23 Electrolytic Condenser Vol- 4MF-6volt 500 ohms.

ume Control. 32 Resistor 6.8K ohms. 34 Battery 9 volts. 35- i 1,000ohms. 41 Resistor 47 ohms. 42.. Electrolytic Condenser 50 mf.15 Volts.47--." Output transformer 65 and 66.-.- Single pole Single throw 500ohms to 3.2 ohms.

. switches.

I claim:

1. In a multistage amplifying circuit, a source of potential havingpositive and negative poles, first and second transistors each having abase, collector, and emitter, a transformer having primary and secondarywindings, an input circuit having one end connected to ground and theother end to one end of the primary winding which has its other endconnected to ground, said secondary winding having one end connected tothe base of the first transistor, a resistance connecting the other endof the secondary winding to ground, a capacitor and variable resistanceconnected between said other end of the secondary winding and theemitter of the first transistor, a connection from between the capacitorand the variable resistance to the base of the second transistor, aresistor connected between the positive pole and said other end of thesecondary winding, a resistor connected between the positive pole andthe base of the second transistor, a resistor connected between thepositive pole and the emitter of the second transistor, a capacitorconnected between the emitter of the second transistor and ground, atransformer having a primary bridged between the collector of the secondtransistor and ground and having a secondary output, a ground connectionbetween the collector of the first transistor and ground, and a groundconnection be tween the negative pole and the ground.

2. An amplifying circuit for energization from a source of potentialhaving first and second poles comprising first and second semiconductorunits, each unit having input, output, and common electrodes, a firsttransformer having primary and secondary windings, an input circuitincluding said primary winding, means for coupling one end of saidsecondary winding to the input electrode of the first unit, meansincluding a first resistance for coupling the other end of saidsecondary winding to ground, a first capacitor having one plate coupledto said other end of the secondary winding, means including a variableresistor for intercoupling the other plate of said capacitor to thebutput electrode of said first unit, means for coupling the commonelectrode of said first unit to ground, means for coupling the inputelectrode of said second unit to the common connection between saidcapacitor and said variable resistor, a second resistance intercoupledbetween said other end of the secondary winding and said first pole, athird resistance coupled between said common connection and said firstpole, a fourth resistor coupled between the common electrode of thesecond unit and said first pole, a second capacitor intercoupled betweensaid common electrode of the second unit and ground, a secondtransformer having primary and secondary windings, means including theprimary winding of said second transformer for coupling the outputelectrode of the second unit to ground, and means end of said secondarywinding and ground, means includ ing a second resistor for intercouplingsaid other end of the secondary winding and said first pole, meansinclud-' ing a first capacitor for intercoupling said other end of thesecondary winding and the input electrode of said second transistor,means including a variable resistance intercoupled between the outputelectrode of said first.

transistor and the common connection between said first capacitor andthe input electrode of said second transistor, means including a thirdresistor intercoupled between said common connection and said firstpole, means including a fourth resistor intercoupled between the commonelectrode of said second transistor and said first pole, means includinga capacitor intercoupled between the common electrode of said secondtransistor and ground, means for intercoupling said primary windingbetween the output electrode of said second transistor and ground, andmeans for coupling said second pole to ground.

4. A two-stage transistor amplifier circuit for use between thesecondary winding of an input transformer and the primary winding of anoutput transformer, said circuit being energized from a potential sourcehaving first and second poles, comprising input, output and commonelectrodes in each transistor, means for intercoupling one end of saidsecondary winding and the input electrode of the first transistor, meansfor intercoupling the common electrode of the first transistor andground, means including a first transistor for intercoupling the otherend of said secondary winding and ground, means including a secondresistor for intercoupling said other end of the secondary winding andsaid first pole, resistance means intercoupled between the outputelectrode of said first transistor and said first pole, a capacitorintercoupled between the other end of said secondary winding and apointon said resistance means, means for coupling the input electrode ofsaid second transistor to a point on said resistance means, a thirdresistor intercoupled between the common electrode of said secondtransistor and said first pole, means including a capacitor intercoupledbetween the common electrode of said second transistor and ground, meansfor intercoupling said primary winding between the output electrode ofsaid second transistor and ground, and means for coupling said secondpole to ground.

5. In a transistorized amplifier constructed for energization from asource of potential having positive and negative poles, at least twotransistors each having input, output and common electrodes, an inputtransformer including a secondary Winding and an output transformerincluding a primary winding, means for coupling one end of saidsecondary winding to the input electrode of said first transistor, afirst resistor coupled between the other end of said secondary windingand a reference circuit connection, a second resistor coupled betweensaid other end of the secondary winding and a first one of said poles,resistance means coupled between the output electrode of said firsttransistor and said first pole, means for coupling the common electrodeof said first transistor to said reference connection, a first capacitorcoupled between said other end of the secondary winding and a point onsaid resistance means, means for coupling the input electrode of saidsecond transistor to a point on said resistance means, a third resistorcoupled between the common electrode of said second transistor and saidfirst pole, a second capacitor coupled to said common electrode of saidsecond transistor to effectively bypass said third resistor, means forcoupling said primary winding between the output electrode of the secondtransistor and said reference connection, and means for coupling theother of said poles to said reference connec tion.

6. For energization from a source of potential having first and secondpoles, an amplifying circuit coupled between 21 pair of input conductorsand a pair of output conductors including at least two transistors, eachtransistor having input, common, and output electrodes, means forcoupling both the common electrode of the first transistor and one ofsaid output conductors to a reference connection, means for coupling oneof said input conductors to the input electrode of said firsttransistor, means for coupling the other of said input conductorsthrough a first resistor to said reference connection and through asecond resistor to said first pole, resistance means, of which at leasta portion is variable, coupled between the output electrode of saidfirst transistor and said first pole, a first capacitor coupled betweensaid other input conductor and said resistance means, means for couplingthe input electrode of said second transistor to said resistance means,means for coupling the output electrode of said second transistor to theother of said output conductors, a third resistor 55 coupled between thecommon electrode of said second transistor and said first pole, a secondcapacitor coupled to said common electrode of the second transistor, andmeans for intercoupling said reference connection with said second poleof the potential source.

References Cited by the Examiner UNITED STATES PATENTS OTHER REFERENCESGarner, Transistor Guitar Amplifier, Radio and Television News, November1953, pages 74, 75, 192.

ROY LAKE, Primary Examiner.

WILLIAM C. COOPER, NATHAN KAUFMAN,

Examiners.

2. AN AMPLIFYING CIRCUIT FOR ENERGIZATION FROM A SOURCE OF POTENTIALHAVING FIRST AND SECOND POLES COMPRISING FIRST AND SECOND SEMICONDUCTORUNITS, EACH UNIT HAVING INPUT, OUTPUT, AND COMMON ELECTRODES, A FIRSTTRANSFORMER HAVING PRIMARY AND SECONDARY WINDING, AN INPUT CIRCUITINCLUDING SAID PRIMARY WINDING, MEANS FOR COUPLING ONE END OF SAIDSECONDARY WINDING TO THE INPUT ELECTRODE OF THE FIRST UNIT, MEANSINCLUDING A FIRST RESISTANCE FOR COUPLING THE OTHER END OF SAIDSECONDARY WINDING TO GROUND, A FIRST CAPACITOR HAVING ONE PLATE COUPLEDTO SAID OTHER END OF THE SECONDARY WINDING, MEANS INCLUDING A VARIABLERESISTOR FOR INTERCOUPLING THE OTHER PLATE OF SAID CAPACITOR TO THEOUTPUT ELECTRODE OF SAID FIRST UNIT, MEANS FOR COUPLING THE COMMONELECTRODE OF SAID FIRST UNIT TO GROUND, MEANS FOR COUPLING THE INPUTELECTRODE OF SAID SECOND UNIT TO THE COMMON CONNECTION BETWEEN SAIDCAPACITOR AND SAID VARIABLE RESISTOR, A SECOND RESISTANCE INTERCOUPLEDBETWEEN SAID OTHER END OF THE SECONDARY WINDING AND SAID FIRST POLE, ATHIRD RESISTANCE COUPLED BETWEEN SAID COMMON CONNECTION AND SAID FIRSTPOLE, A FOURTH